China is coal-rich and oil-lean, so coal liquefaction not only has great environmental significance, but also has the strategic significance of energy security in response to today's contradiction between supply and demand and implementing the policy of energy saving and emission reduction. Further understanding of the coal resources, comprehensive utilization of coal resources and petroleum resources, and construction of integrated development model for petroleum chemical industry and coal chemical industry, are effective measures to realize the sustainable development of the petrochemical industry, to improve the efficiency of resource use and to promote resource security.
Oil-coal co-hydrotreating processing has the following characteristics: (1) High conversion: the conversion of oil-coal co-processing is more than 90%, far greater than the conversion of a single coal hydro-liquefaction or heavy oil hydrocracking. (2) Existing a synergistic effect: Due to the existence of synergistic effect between coal and heavy oil, the total amount of generated oil from oil-coal co-processing is higher than that of a single coal hydro-liquefaction or heavy oil hydrocracking, and the presence of coal not only prevents the catalyst from carbon deposition but also is conducive to removal of metallic impurities from heavy oil. Thereby, the service life of the catalyst is prolonged, and the economic benefit of the enterprise is improved. (3) The high oil yield: Compared to coal liquefaction, the oil yield of production plant is greatly improved as a result of once-through operation mode. (4) The hydrogen consumption is relatively low: Compared to coal direct liquefaction, chemical hydrogen consumption is low and hydrogen utilization rate is greatly improved for oil-coal co-processing, which is beneficial to reduce energy consumption and equipment investment. (5) The pulverized coal added not only is the feedstock, but also can adsorb the resin and asphaltene and become the coke carrier to effectively prevent the coke formation in the reaction system. (6) The quality of oil is good: Compared to coal direct liquefaction, the products with higher hydrogen content and lower aromatics content and are easily processed into qualified gasoline and diesel. (7) Strong market competitiveness: Increased conversion and productivity, lower hydrogen consumption, better oil quality and other positive factors, make oil-coal co-processing lower costs in many aspects than that of direct coal liquefaction.
On the other hand, now the main problems of crude oil processing are low light oil yield and high coke formation, which leads to poor utilization and efficiency of crude oil. Especially China's imported crude oil is mostly heavy oil with higher sulfur content, which will produce average 30% coke in the oil refining process. Many local refineries directly import heavy oil from abroad for processing, while the processing technologies of heavy oil has a large room to improve.
As a result, oil refining enterprises solve the problems of heavy oil upgrading by using oil-coal co-processing technology, and moreover, oil-coal co-processing greatly improves the light oil yield and the economic efficiency of enterprises.
Oil and coal slurry bed hydrotreating technology is a kind of coal to oil technology based on heavy oil hydrocracking process in petroleum industry. In oil-coal co-processing, coal slurry made of pulverized coal and heavy oil is hydrotreated in high temperature and high pressure to produce liquid fuel oil. Because coal has the characteristic of “aromatic rich and hydrogen deficient” while “aromatic deficient and hydrogen rich” for heavy oil and residual oil, by adjusting the co-processing conditions, it can greatly reduce the severity of direct coal liquefaction process.
At present, the domestic oil-coal co-processing mainly comprises the following steps: (1) Pulping for coal, catalyst and heavy oil; (2) Coal slurry is preheated into the reactor; (3) The reaction products are separated into gaseous substance, light oil, water and heavy mixture. Gaseous substance is purified through pressure swing adsorption by PSA for purified hydrogen that is back to reactor for recycle utilization, and the remaining gas is purified and used as fuel. Separated light oil and water are further carried on oil-water separation to get purer light oil and water; (4) A mixture of heavy oil is sent to the distillation tower to get crude oil and bottom product; (5) The mixed crude oil and light oil are upgraded to gasoline, kerosene, diesel oil, fuel oil and other liquid fuels; (6) The bottom product is processed into asphalt paving materials. The technique is combined the “aromatic rich and hydrogen deficient” coal, “aromatic deficient and hydrogen rich” residue with catalysts. By adjusting the co-processing conditions, it greatly reduces the severity of direct coal liquefaction process and produces liquid fuel and asphalt paving materials. To a certain degree, it improves the economy of direct liquefaction of coal.
There are two sections of oil-coal co-processing in abroad. Coal slurry made by heavy oil and coal with the ratio of 1:2 to 11:9 are preheated with hydrogen into a ebullated bed reactor to 435-445° C. and 15-20 MPa in the presence of Co—Mo/Al2O3 catalyst for hydrocracking reaction, and the reaction products entere the second period of ebullated bed reactor in the presence of Ni—Mo/Al2O3 catalysts for hydrogenation to removal of sulfur, nitrogen, oxygen and heavy metals (Ni, V). The gas is obtained after separating from the product, and ammonia and sulfur in the gas are recovered and then the hydrogen is recycle used. Liquid products enter atmosphere distillation and vacuum distillation to obtain target products. The process has the characteristics of simple process, reliable technology, wide range of raw material adaptation, high oil yield, high rate of metal removal of the heavy oil, etc. The technology has less hydrogen consumption and higher hydrogen utilization rate because of with no ash removal section.
But the above oil-coal co-processing will inevitably encounter such a problem: because of high viscosity of heavy oil as solvent oil, in order to ensure good stability and liquidity of coal slurry, it usually needs to reduce solid content of coal slurry. The reduction of solid content of coal slurry will affect the liquid fuel yield, and the highest solid content can reach about 35% for existing technologies.